Odorant binding protein

BmorPBP structure and function

This protein was first identified in the B. mori male antennae by Krieger et al. in 1996 ^[4] . It has 164 amino acids that forms 6-7 alpha helices. Three 3 disulfide bonds formed by 6 cystein residues tied four helices. As expected from a soluble protein, its surface is covered with charged residues.

BmorPBP ligand and ligand binding

The protein natural ligand is the moth pheromone bombykol. However, it was demonstrated that other molecules can also bound to the protein cavity ^[5]. The interaction with the ligand is beeing made by 4 alpha helices in the core of the protein, which form the binding cavity ^[6]. The hydroxyl group of the pheromone bombykol forms a hydrogen bond with the sidechain of Ser56 (O–O distance of 2.8 Å).

Protein conformations

The PBP has two pH-dependent conformations, with different ligand affinities: the "open form" (A) and the "close form" (B) ^[7]. The pheromone bombykol, and the seventh α-helix loacated in the C terminus of the protein compete for the binding site. In a nuetral pH (6.5-7) the protein is in the "open form" (A), in which the

The transition between the two conformation is taking place between 6-5 pH.

The C terminus of the protein bears mostly nonpolar amino acids. Yet on the surface of the helix there are three exceptional amino acids: Asp-132, Glu-137, and Glu-141, which are conserved in moth PBP ^[8]. Of these, residues Asp-132 and Glu-141 triggers the formation of the α-helix upon protonation at low pH. This what is causing the ejacullate of the ligand from the binding pocket, which is replaced by the formated alpha helix^[9].

• "open form" (A)
• "close form" (B)

Studies on other Lepidopterans that show a simmilar pH dependent conforamation suggests that this model is a genral model moth PBP. Nonetheless, the enourmas diversity among insects is not allowing us to assume this model is true for all insects.

Receptor activation

Two theories have been propsed for the activation of the odorant receptors located on the dendrtirte membrane. One theory suggests that the pheromone-PBP complex is needed for the receptor activation, while the second theory argue that the pheromone itself is sufficient for the activation of the receptor.

• Activation by the complex pheromone-PBP

• Activation by the pheromone alone

This model is supported by the pH dependent conforamtion transition, that is described above.

The surface of the dendrite is negatively charged ^[10], which cause the accumulation of positively charged kations near the membrane surface (20-50 nm), thereby inducing a low pH environment near the dendrite membrane kaissling 2009

Leal WS, et al. (2005) Kinetics and molecular properties of pheromone binding and release. Proc Natl Acad Sci USA 102(15):5386–5391.

The BPB ligand is the moth pheromone